What is Contraction: Definition and 708 Discussions

Muscle contraction is the activation of tension-generating sites within muscle cells. In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in muscle length, such as when holding a heavy book or a dumbbell at the same position. The termination of muscle contraction is followed by muscle relaxation, which is a return of the muscle fibers to their low tension-generating state.Muscle contractions can be described based on two variables: length and tension. A muscle contraction is described as isometric if the muscle tension changes but the muscle length remains the same. In contrast, a muscle contraction is isotonic if muscle tension remains the same throughout the contraction. If the muscle length shortens, the contraction is concentric; if the muscle length lengthens, the contraction is eccentric. In natural movements that underlie locomotor activity, muscle contractions are multifaceted as they are able to produce changes in length and tension in a time-varying manner. Therefore, neither length nor tension is likely to remain the same in muscles that contract during locomotor activity.
In vertebrates, skeletal muscle contractions are neurogenic as they require synaptic input from motor neurons. A single motor neuron is able to innervate multiple muscle fibers, thereby causing the fibers to contract at the same time. Once innervated, the protein filaments within each skeletal muscle fiber slide past each other to produce a contraction, which is explained by the sliding filament theory. The contraction produced can be described as a twitch, summation, or tetanus, depending on the frequency of action potentials. In skeletal muscles, muscle tension is at its greatest when the muscle is stretched to an intermediate length as described by the length-tension relationship.
Unlike skeletal muscle, the contractions of smooth and cardiac muscles are myogenic (meaning that they are initiated by the smooth or heart muscle cells themselves instead of being stimulated by an outside event such as nerve stimulation), although they can be modulated by stimuli from the autonomic nervous system. The mechanisms of contraction in these muscle tissues are similar to those in skeletal muscle tissues.

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  1. M

    Spaceship length contraction problem

    For this problem, Does someone please know that won't the cabin, the person, and the clothing all shrink by the same proportion in the direction of travel so if you brought smaller cloths or a smaller cabin than you normally would then you may not be able to fit in? So you should not buy a...
  2. Imya

    I Transition of spherically symmetric gravitation

    Outside a static spherically symmetric body in vacuum lies the Schwarzschild metric. However, when observing a contractive sphere from a point near the sphere, it no longer appears spherical. Its opposite end appears stretched for an observer stationary relative to the center of the sphere. In...
  3. curiousburke

    I Relativity of Simultaneity and Length Contraction

    This has probably been discussed/explained many times before so I apologize in advance. Please just direct me to the relevant thread if it has. In Morin, "Special Relativity For the Enthusiastic Beginner", he explains the loss of simultaneity and specifically the "rear clock ahead" effect. If...
  4. A

    I Perceive Length Contraction VS Terrell Penrose

    This is a more of a "can you confirm" what I am thinking here; I think I understand what is going on, but just want to make sure. At the end of this post, I reference the two selected websites at the times mentioned that compare "Length Contraction" verses "Terrell Penrose Rotation." My...
  5. P

    I Having trouble understanding Tensor Contraction

    I'm having trouble understanding tensor contraction. So for example, for something like AuvBvu, would this equal to some scalar?
  6. S

    I Orthonormal basis expression for ordinary contraction of a tensor

    I'm reading Semi-Riemannian Geometry by Stephen Newman and came across this theorem: For context, ##\mathcal{R}_s:Mult(V^s,V)\to\mathcal{T}^1_s## is the representation map, which acts like this: $$\mathcal{R}_s(\Psi)(\eta,v_1,\ldots,v_s)=\eta(\Psi(v_1,\ldots,v_s))$$ I don't understand the...
  7. N

    I Calculate Length Contraction for Accelerated Motion to Proxima Centauri

    Let's assume a spaceship traveling from the Earth to the Proxima Centauri with constant acceleration g = 9.81 m/s2. The ship is accelerating the first half of the trajectory and decelerating the second half. I calculated the velocity profile from the Earth reference: The travel time on...
  8. M

    B Length Contraction: Exploring the Possibility of Black Holes at High Speeds

    If I am traveling in my spacecraft at .99999999 percent the speed of light past a star, then according to the equation of length contraction a star with 4x or more solar mass would contract along the line of motion according to my frame of reference by an amount of over Length naught (8*10-E)...
  9. T

    I Time Dilation & Length Contraction: Graphically Explained

    The thread "twin paradox without math" inspired me try to find fully graphical solution of length contraction. Here is the result: Graph is 3D as I think that is ilustrative to make all in one graph, but I am sure that it is possible to do it in 2 or 3 2D slices. X and Y are spatial axes, T is...
  10. M

    I Lorentz Contraction & Twin Paradox

    The twin paradox is connected to the special relativity but I wonder simply if one might construct the paradox (or something very similar) based on the Lorentz’ (and FitzGerald) work alone? Several ingredients in the paradox, time dilation and Lorentz contraction, are often mentioned with...
  11. L

    I Relativistic length contraction

    Hello! I have a question. If there is a wooden rod which burns certain time, and an astronaut inside rocket lights two such rods: one oriented along the rocket's length and the other goes across the rocket, and an observer see the rocket passing by with a relativistic speed. Will the observer...
  12. K

    I Newton's Gravity & Lorentz Contraction: Is Modification Needed?

    Newton's gravity depends on the euclidean distance between two masses. Two comoving frames will have different values of length between masses so the forces will be different in two frames. Is it enough to prove that the gravity rule has to be modified?
  13. S

    I An issue with length contraction

    I have read that length contraction is real and not just an illusion. However, when I compare an object at rest with an observer and the object in motion relative to another observer, I see that instead of there being a real length contraction, there is simply a difference in the part of the...
  14. E

    I Time difference caused by length contraction

    The goal is to calculate the difference in the passage of time between two ends of a cabin which is accelerating upwards due to length contraction. To help in the calculations we can consult: https://arxiv.org/pdf/1807.05338.pdf There is an old problem called the 4/3 problem which has been...
  15. G

    I Calculate Contraction 2nd & 4th Indices Riemann Tensor

    How to calculate the contraction of second and fourth indices of Riemann tensor?I can only deal with other indices.Thank you!
  16. DavidBalut

    I The Theoretical Minimum: Length Contraction and Time Dilation

    Hello, My name is Dave and I'm a physics major at UIUC. It looks like I will be taking the special relativity course (phys 225) this fall. I've always been fascinated by the theory so I decided to get a head start with Lenny and Art's perspective on it. My first head-scratching moment came in...
  17. Kairos

    I Distance Contraction: Is D or D*sqrt(1-(v/c)^2) Traveled?

    Two planets are immobile one with respect to the other and distant of D. A rocket passes from one to the other at high speed v. Is the distance really traveled by the rocket between these planets D or only D*sqrt(1-(v/c)^2) ?
  18. K

    I Length contraction in General Relativity

    In GR, a free falling object when viewed by a distant observer appears to be length contracted and slows down as it approaches the event horizon of a black hole. The length contraction piece, however, seems counterintuitive. I would have thought that the leading edge of the object would...
  19. PainterGuy

    Time dilation and length contraction for a rocket

    Hi, It's not a homework but still thought to post it here as advised in the past. A rocket is going to leave Earth's surface and it is decided that a data pulse encoding emission time of pulse will be sent every second from Earth station to the rocket, and the rocket would do the same. The...
  20. guyvsdcsniper

    Find the contraction of angles seen by an observer

    I am trying to follow the work to this question but am stumped at steps 3 and 4. I am confused as to where the cos^2(90+θ) comes from? I can see it is used to invoke sin into the equation since we have that value. Is it because we are only measuring the x-component of the movement, so we need...
  21. guyvsdcsniper

    B Check my understanding of Time Dilation and Length Contraction

    I just started learning about Special Relativity and have come upon the topics of Time dilation and Length contraction. Its a bit abstract for me and I just want to cross ref my knowledge here and see if someone can tell me if I am understanding this correctly. I've attached an excerpt of a...
  22. alexandrinushka

    B Any plausible explanation for dynamical length contraction?

    I am new here, so pardon my ignorance. First of all, I am aware of the impossibility to distinguish experimentally between SR (Special Relativity) and LET (Lorentz Ether Theory). I know there is a PF policy article on LET and the Block Universe. I must admit though that LET is more appealing...
  23. S

    I Because of length contraction, does absolute position exist?

    I know that I have heard that GR proves that there is no absolute time, so does this go for location too? If so, could it be said that particles have a superposition in position/location the same way that particles do in QM?
  24. O

    I Linear Accelerator Length Contraction

    I am trying to understand the effect of relativistic length contraction on the electron bunches in a linear accelerator. Figure B is for nonrelativistic speeds, successive cylinder lengths are progressively longer. However, wikipedia says "At speeds near the speed of light, the incremental...
  25. alan123hk

    B The Paradox of Relativity Length Contraction

    A rigid rod with length ## l_0## slides on a smooth and flat tabletop along the length at speed of ## ~\frac {\sqrt{3}}{2}c~ ##, there is a hole of width ##~l_0~##on the table. The observer who is stationary relative to the desktop thinks that the length of the rigid rod ##~ l=l_o...
  26. S

    B Is length contraction (Lorentz transformation) an illusion or real?

    My textbook (from first year university physics) says that length contraction is actually real. But how can it be real when two different observers can measure two different lengths? For example, if I am in a spaceship going close to the speed of light relative to people on Earth, they will...
  27. M

    B Distance Length Contraction at Light Speed

    I know it's not possible to travel at light speed so this is just theoretical. As I understand it at relativistic speeds the distance you need to cover to travel to a destination are length contracted. If you were traveling at light speed is the distance between you and any object ever zero?
  28. O

    I Compute Length Contraction: Is ##l=1## a Possible Model?

    Sorry, I accidentally posted this while typing it up, then when I finished typing it I found the mistake that made me come here to make this thread. I'll still write it up just to make the thread somewhat useful, and then ask my follow-up question at the end I'm picking units so the speed of...
  29. R

    I Length contraction and time dilation

    A long time ago, I was very impressed by a lecture on elementary special relativity which showed in simple math how the correct conclusion to the null results of the Michelson-Morley experiment were obtained by concluding a constant c but then adding a length contraction and time dilation. I...
  30. Kaguro

    Deriving length contraction using spacetime

    Deriving time dilation was easy: Imagine two events in frame O' at the same location. ##ds^2 = -c^2 dt'^2## The same viewed in O frame is: ##ds^2 = dx^2+dy^2 + dz^2 - c^2 dt^2## ##\Rightarrow dx^2+dy^2 + dz^2 - c^2 dt^2 = -c^2 dt'^2## ##\Rightarrow (\frac{dx}{dt})^2+(\frac{dy}{dt})^2+...
  31. kolleamm

    Muscle contraction question about myosin

    I've been doing research on how muscles work using the page below and overall I get the general idea, however I still have some questions. Why does ADP and Pi detach from the myosin head during the power stroke and why does the attachment of ATP to the myosin head cause the myosin head to detach...
  32. greg_rack

    Special relativity (length contraction, velocity composition) problem

    First, I calculated the velocity of the object with regards to earth, which is: $$v'=\frac{V-v}{1-\frac{vV}{c^2}}=0.34c$$ Now, the problem is solved if I consider the length ##l=45m##(so by calculating the ##\gamma## factor with ##v'##) to be the proper one... but since it's measured by the...
  33. D.S.Beyer

    B SR vs GR - Visual Difference between Length Contraction

    The 2 Bowling Balls Ball(a) & Ball(b) (a) is in acceleration of 10m/s^2 (b) is in at fixed position in a gravitational field where g=10m/s^2 In both cases the observer is: - perpendicular to the vector of acceleration - distant enough to be in empty flat space Question : In an instantaneous...
  34. F

    Rocket moving away from the Earth

    I first began to identify the various events in the problem. I call the rocket ##S'## and the Earth ##S##. Sending signal Tail signal reflection Head signal reflection Tail signal return Head signal return For the Earth I know that: ##t_4 = T## ##t_5 = T + \Delta T## Since the two events...
  35. LCSphysicist

    Lorentz contraction and Spacetime diagram

    Hello, i can't understand how does the author found this expression relating ##x_{c}## and v. I already tried by a lot of geometrical ways, knowing that the tangent of the angle between the dotted line and the x-axis should be v, but the results are illogical. Could you help me? I am start to...
  36. sandmanvgc

    Force Exerted by fluid on a Bend, an Enlargement, or a Contraction

    My attempt to try and convert book equation to FE manual equation in 3rd pic Fx = QpV1 + P1A1 - QpV2Cosa - P2A2Cosa Fx = P1A1 - P2A2Cosa+ Qp(V2Cosa +V1) P1A1 - P2A2Cos(a) - Fx = -Qp(V2Cosa +V1)
  37. J

    B Experimental Confirmation Length Contraction Velocity Addition

    What if any is the experimental confirmation for length contraction and the velocity addition formula?
  38. U

    I Length Contraction & Time Dilation: Proving It

    I have been able to prove to myself that, based on Einstein's two postulates and the the Pythagorean theorem, that time dilates. From here how do I prove that length contracts? (All of this observing a frame that is moving relative to the proper frame at uniform velocity.)
  39. T

    B On time dilation and length contraction

    Hello there.About time dilation, could we provide a derivative of time in relation to one of the coordinates of the manifold we have taking time as a function and get something as a result?Or its integral?And about time dilation we have the formula that gives it between two clocks and an...
  40. S

    B Light Clock & Length Contraction in GR

    Hi, can i use a light clock made out of mirrors a distance appart to measure whether there is length contraction in different regions of spacetime? If the clock speeds up then the distance between the mirrors decreased. If the clock slows down the distance between mirrors increased.
  41. X

    B Time Dilation & Length Contraction: Physics Explained

    So basically i know almost nothing about physics but i have this one curiosity and i hope you can help me ahah. For what i understand if you could move at the speed of light time would stop for you and you would see the whole universe age in a blink of an eye. But what if you could stand...
  42. Stephen Bulking

    Length contraction and Time dilation for LIGHT?

    Radio wave travels at the speed of light 3x10^8 (m/s) Converting the distance to meter: 1.3 x 3.1x 10^16 = 4.03x10^16m The time it takes in our Earth frame of reference is: 4.03x10^16m/3x10^8 (m/s) = 4.26 years The answer is B But wouldn't the time in light's frame of reference be 0 and it's...
  43. K

    I Vanishing of Contraction with Metric Tensor

    This question is probably silly, but suppose I have a contraction of the form ##g_{\mu \nu} C^{\mu \nu} = 0## where ##C^{\mu \nu}## is a tensor* and ##g_{\mu \nu}## is the metric tensor. Can I say that it must vanish for any ##g_{\mu \nu}##, and since in the most general case all ##g_{\mu \nu}##...
  44. Flying_Dutchman

    I Length Contraction: Traveling in a Space Ship

    Suppose I'm traveling inside a spaceship at speed comparable to light between two points A and B. According to me the distance between the two points will be shortened due to length contraction. But actually my spaceship passes through every point between A and B so the distance measured by...
  45. P J Strydom

    B Length contraction and Time dilation calculation

    I was absent for a while due to personal constraints but I did keep myself busy with the Time dilation equation some member sent me a while back. I decided to set a time limit for myself to learn and understand time Dilation and length contraction, which must be before December 2020, or I will...
  46. Jason Bennett

    Exploring the Inönü-Wigner Contraction of Poincaré $\oplus$ $\mathfrak{u}$(1)

    Please see https://physics.stackexchange.com/questions/552410/inönü-wigner-contraction-of-poincaré-oplus-mathfraku1Inönü-Wigner contraction of Poincaré \oplus \mathfrak{u}(1) Metric = (-+++), complex $i$'s are ignored. _____________________________________________________________________ The...
  47. P

    Longitude contraction / time dilatation

    I get the same values for both observers, which is not nice I think.
  48. G

    B Length Contraction Thought Experiment: Spot Mistake/Wrong Assumption

    Alice travels in a spaceship, which she measures to be L. The spaceship is moving with velocity v relatively to Bob. Alice makes a light beam traveling along the spaceship and measures the time interval it takes to go from one end to another, ΔtA. So, equation (1): L = c × ΔtA From Bob's...
  49. Physics lover

    Chemistry Ring contraction in a diazotisation reaction

    I know HNO2 favours diazotisation.So amine reacted and formed diazorised product with N+=NCl- which leaves and gives a carbocation to which OH- attacks.So according to me the answer should be cyclohexane-1,2-diol. But in the answer key,cyclopentane carboxyaldehyde was given as answer. I know it...
  50. M

    I Where is Contraction Point in Special Relativity?

    Generally speaking, when a simple contraction occurs there is a contraction point. Length contraction in special relativity appears to be a simple contraction, and hence there should be a contraction point. Where is this contraction point located?
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